Chemotrophs are organisms that obtain energy by the oxidation of electron donors in their
environments.
These molecules can be organic (chemoorganotrophs) or inorganic (chemolithotrophs).
The chemotroph designation is in contrast to phototrophs, which use solar energy.
Chemotrophs can be either autotrophic or heterotrophic.
Chemotrophs are found in ocean floors where sunlight cannot reach them because they are
not dependent on solar energy.
Ocean floors often contain underwater volcanos that can provide heat as a substitute for
sunlight's warmth.
== Chemoautotroph == Chemoautotrophs (or chemotrophic autotroph)
(Greek: Chemo (χημία) = chemical, auto (αὐτός) = self, troph (τροφιά)
= nourishment), in addition to deriving energy from chemical reactions, synthesize all necessary
organic compounds from carbon dioxide.
Chemoautotrophs use inorganic energy sources such as hydrogen sulfide, elemental sulfur,
ferrous iron, molecular hydrogen, and ammonia.
Most chemoautotrophs are extremophiles, bacteria or archaea that live in hostile environments
(such as deep sea vents) and are the primary producers in such ecosystems.
Chemoautotrophs generally fall into several groups: methanogens, halophiles, sulfur oxidizers
and reducers, nitrifiers, anammox bacteria, and thermoacidophiles.
An example of one of these prokaryotes would be Sulfolobus.
Chemolithotrophic growth can be dramatically fast, such as Hydrogenovibrio crunogenus with
a doubling time around one hour.The term "chemosynthesis", coined in 1897 by Wilhelm Pfeffer, originally
was defined as the energy production by oxidation of inorganic substances in association with
autotrophy - what would be named today as chemolithoautotrophy.
Later, the term would include also the chemoorganoautotrophy, that is, it can be seen as a synonym of chemoautotrophy.
== Chemoheterotroph == Chemoheterotrophs (or chemotrophic heterotrophs)
(Gr: Chemo (χημία) = chemical, hetero (ἕτερος) = (an)other, troph (τροφιά)
= nourishment) are unable to fix carbon to form their own organic compounds.
Chemoheterotrophs can be chemolithoheterotrophs, utilizing inorganic energy sources such as
sulfur or chemoorganoheterotrophs, utilizing organic energy sources such as carbohydrates,
lipids, and proteins.
Most animals and fungi are examples of chemoheterotrophs.
== Iron- and manganese-oxidizing bacteria ==
In the deep oceans, iron-oxidizing bacteria derive their energy needs by oxidizing ferrous
iron (Fe2+) to ferric iron (Fe3+).
The electron conserved from this reaction reduces the respiratory chain and can be thus
used in the synthesis of ATP by forward electron transport or NADH by reverse electron transport,
replacing or augmenting traditional phototrophism.
In general, iron-oxidizing bacteria can exist only in areas with high ferrous iron concentrations,
such as new lava beds or areas of hydrothermal activity.
Most of the ocean is devoid of ferrous iron, due to both the oxidative effect of dissolved
oxygen in the water and the tendency of bacteria to take up the iron.
Lava beds supply bacteria with ferrous iron straight from the Earth's mantle, but only
newly formed igneous rocks have high enough levels of ferrous iron.
In addition, because oxygen is necessary for the reaction, these bacteria are much more
common in the upper ocean, where oxygen is more abundant.
What is still unknown is how exactly iron bacteria extract iron from rock.
It is accepted that some mechanism exists that eats away at the rock, perhaps through
specialized enzymes or compounds that bring more FeO to the surface.
It has been long debated about how much of the weathering of the rock is due to biotic
components and how much can be attributed to abiotic components.
Hydrothermal vents also release large quantities of dissolved iron into the deep ocean, allowing
bacteria to survive.
In addition, the high thermal gradient around vent systems means a wide variety of bacteria
can coexist, each with its own specialized temperature niche.
Regardless of the catalytic method used, chemoautotrophic bacteria provide a significant but frequently
overlooked food source for deep sea ecosystems - which otherwise receive limited sunlight
and organic nutrients.Manganese-oxidizing bacteria also make use of igneous lava rocks
in much the same way; by oxidizing manganous manganese (Mn2+) into manganic (Mn4+) manganese.
Manganese is more scarce than iron oceanic crust, but is much easier for bacteria to
extract from igneous glass.
In addition, each manganese oxidation donates two electrons to the cell versus one for each
iron oxidation, though the amount of ATP or NADH that can be synthesised in couple to
these reactions varies with pH and specific reaction thermodynamics in terms of how much
of a Gibbs free energy change there is during the oxidation reactions versus the energy
change required for the formation of ATP or NADH, all of which vary with concentration,
pH etc.
Much still remains unknown about manganese-oxidizing bacteria because they have not been cultured
and documented to any great extent.
== Flowchart ==
Autotroph Chemoautotroph
Photoautotroph Heterotroph
Chemoheterotroph Photoheterotroph
== See also ==
Chemosynthesis Lithotroph
== Notes